Effects of Elevated Ultraviolet-B Radiation on Productive Tillers, Spikelet Sterility and Grain Characteristics of Southern US Rice (Oryza sativa L.) Cultivars

Depletion of the stratospheric ozone layer is leading to an increase in ultraviolet-B (UVB) radiation (280-320 nm) reaching the Earth's surface, which can alter crop productivity. The objective of this study was to determine the effects of elevated UVB radiation on rice production with special emphasis on grain characteristics in southern US rice cultivars (six inbred cultivars and three hybrids). Due to the spectral characteristics of the greenhouse glass, plants received no natural UVB radiation; hence, plants were provided with supplemental UVB lighting. Plants were exposed to UVB radiation of 0, 8 (ambient) or 16 (twice ambient) kJ m(-2) day(-1). There was no significant effect of elevated UVB radiation on the number of productive tillers in any cultivar except 'Sierra'. All the cultivars except 'Sierra' showed increased spikelet sterility with an increase in UVB radiation. There were decreases in main-stem panicle length, number of primary branches per panicle, and grain width with an increase in UVB radiation in most of the inbred cultivars. Elevated UVB radiation decreased plant grain yield in all the cultivars, however the per cent decrease in plant grain yield varied among the cultivars (31-79 %). The hybrid 'Clearfield XL729' performed best among the selected southern US rice cultivars under an elevated UVB environment.